When God has sent his angel to me, then I know of a surety.... When God sends his angel to the soul it becomes the one who knows for sure. Not for nothing did God give the keys into St. Peter's keeping, for Peter stands for knowledge, and knowledge is the key that unlocks the door, presses forward and breaks in, to discover God as he is.

Sermon 9, as translated in The Reading and Preaching of the Scriptures in the Worship of the Christian Church (1999) by Hughes Oliphant Old, Ch. 9: The German Mystics, p. 448